The knowledge of the amount of residual propellant available onboard a spacecraft is of prime importance to optimize the use of the spacecraft and more particularly at the end of the spacecraft mission in order to decide whether re-orbit manoeuvres can be performed with a view to placing the spacecraft into a new orbit.
Establishing the quantity of the remaining propellant in the tanks of a spacecraft has been a constant endeavour since the launch of active spacecrafts and different methods have been proposed to that purpose. These known methods generally provide good results in so far as the measurements are performed in a gravity environment of space because it is no longer possible in that case to know with a good accuracy where exactly the propellant is located in the tanks.
The most currently used methods are based on gas law and consist in performing pressure and temperature measurements. The method which gives the best accuracy consists in recording the amount of propellant consumed during a given time period and substracting this amount from the initial contents of the propellant tanks. Such a method gives satisfactory results when applied on short life spacecrafts but it requires the knowledge of the initial propellant mass available. This initial mass can be established in mono-propellant propulsion systems, but however it is a more difficult problem in a bi-propellant propulsion system for the reason that the proportions of the components in the mixing are not known with accuracy.
Another known method, which has been proposed by the applicant, consists in measuring the pressurant gas pressure and using the residual gas to repressurize the propellant tanks. This known method will allow an estimation of the residual amount of propellant to be made with a great accuracy, even in the case of a bi-propellant propulsion system. However, the pressure measurement can be performed at any time during a spacecraft mission and it cannot be guaranteed that, at the time the measurement is performed, there is at least enough propellant in the tanks for re-orbit manoeuvring. In order to place a geostationnary spacecraft into a new orbit, an amount of propellant is required to raise the altitude of the spacecraft. For example, a rough estimate of the propellant necessary to raise the altitude of an Olympus type satellite by 100 km is 1.5 kg approximately. This propellant mass is much smaller than the total amount loaded prior to launch. In order to prevent the risk of prematurely terminating the commercial use of a spacecraft with a view to optimize its performance capabilities, it is essential to get an accurate control of the mass of residual propellant at the end of the operational life of the spacecraft. The present invention seeks to solve the problem as outlined above.